Cookies are allowed within incognito windows, they're just kept in memory and flushed when all incognito windows are close.

Wow, you learn something every day! I don't hit incognito mode much, I hadn't tried going to my gmail or gdocs. So there you have it, when browsing sites you may not be able to trust (like Arvind Narayanan's blog ;-) be sure to do so in an incognito window.

I've been logged into GMail in an incognito window, and done a Google search in another tab on the same window only to have my email address show up at the top of the search as 'logged in' (same with Google Maps).